java.util
Class Timer

A facility for threads to schedule tasks for future execution in a
background thread. Tasks may be scheduled for one-time execution, or for
repeated execution at regular intervals.

Corresponding to each Timer object is a single background
thread that is used to execute all of the timer's tasks, sequentially.
Timer tasks should complete quickly. If a timer task takes excessive time
to complete, it "hogs" the timer's task execution thread. This can, in
turn, delay the execution of subsequent tasks, which may "bunch up" and
execute in rapid succession when (and if) the offending task finally
completes.

After the last live reference to a Timer object goes away
and all outstanding tasks have completed execution, the timer's task
execution thread terminates gracefully (and becomes subject to garbage
collection). However, this can take arbitrarily long to occur. By
default, the task execution thread does not run as a daemon thread,
so it is capable of keeping an application from terminating. If a caller
wants to terminate a timer's task execution thread rapidly, the caller
should invoke the the timer's cancel method.

If the timer's task execution thread terminates unexpectedly, for
example, because its stop method is invoked, any further
attempt to schedule a task on the timer will result in an
IllegalStateException, as if the timer's cancel
method had been invoked.

This class is thread-safe: multiple threads can share a single
Timer object without the need for external synchronization.

This class does not offer real-time guarantees: it schedules
tasks using the Object.wait(long) method.

Implementation note: This class scales to large numbers of concurrently
scheduled tasks (thousands should present no problem). Internally,
it uses a binary heap to represent its task queue, so the cost to schedule
a task is O(log n), where n is the number of concurrently scheduled tasks.

Timer

Timer

public Timer(boolean isDaemon)

Creates a new timer whose associated thread may be specified to
run as a daemon. A deamon thread is called for if the timer will
be used to schedule repeating "maintenance activities", which must
be performed as long as the application is running, but should not
prolong the lifetime of the application.

schedule

Schedules the specified task for repeated fixed-delay execution,
beginning after the specified delay. Subsequent executions take place
at approximately regular intervals separated by the specified period.

In fixed-delay execution, each execution is scheduled relative to
the actual execution time of the previous execution. If an execution
is delayed for any reason (such as garbage collection or other
background activity), subsequent executions will be delayed as well.
In the long run, the frequency of execution will generally be slightly
lower than the reciprocal of the specified period (assuming the system
clock underlying Object.wait(long) is accurate).

Fixed-delay execution is appropriate for recurring activities
that require "smoothness." In other words, it is appropriate for
activities where it is more important to keep the frequency accurate
in the short run than in the long run. This includes most animation
tasks, such as blinking a cursor at regular intervals. It also includes
tasks wherein regular activity is performed in response to human
input, such as automatically repeating a character as long as a key
is held down.

schedule

Schedules the specified task for repeated fixed-delay execution,
beginning at the specified time. Subsequent executions take place at
approximately regular intervals, separated by the specified period.

In fixed-delay execution, each execution is scheduled relative to
the actual execution time of the previous execution. If an execution
is delayed for any reason (such as garbage collection or other
background activity), subsequent executions will be delayed as well.
In the long run, the frequency of execution will generally be slightly
lower than the reciprocal of the specified period (assuming the system
clock underlying Object.wait(long) is accurate).

Fixed-delay execution is appropriate for recurring activities
that require "smoothness." In other words, it is appropriate for
activities where it is more important to keep the frequency accurate
in the short run than in the long run. This includes most animation
tasks, such as blinking a cursor at regular intervals. It also includes
tasks wherein regular activity is performed in response to human
input, such as automatically repeating a character as long as a key
is held down.

IllegalStateException - if task was already scheduled or
cancelled, timer was cancelled, or timer thread terminated.

scheduleAtFixedRate

public void scheduleAtFixedRate(TimerTask task,
long delay,
long period)

Schedules the specified task for repeated fixed-rate execution,
beginning after the specified delay. Subsequent executions take place
at approximately regular intervals, separated by the specified period.

In fixed-rate execution, each execution is scheduled relative to the
scheduled execution time of the initial execution. If an execution is
delayed for any reason (such as garbage collection or other background
activity), two or more executions will occur in rapid succession to
"catch up." In the long run, the frequency of execution will be
exactly the reciprocal of the specified period (assuming the system
clock underlying Object.wait(long) is accurate).

Fixed-rate execution is appropriate for recurring activities that
are sensitive to absolute time, such as ringing a chime every
hour on the hour, or running scheduled maintenance every day at a
particular time. It is also appropriate for for recurring activities
where the total time to perform a fixed number of executions is
important, such as a countdown timer that ticks once every second for
ten seconds. Finally, fixed-rate execution is appropriate for
scheduling multiple repeating timer tasks that must remain synchronized
with respect to one another.

scheduleAtFixedRate

Schedules the specified task for repeated fixed-rate execution,
beginning at the specified time. Subsequent executions take place at
approximately regular intervals, separated by the specified period.

In fixed-rate execution, each execution is scheduled relative to the
scheduled execution time of the initial execution. If an execution is
delayed for any reason (such as garbage collection or other background
activity), two or more executions will occur in rapid succession to
"catch up." In the long run, the frequency of execution will be
exactly the reciprocal of the specified period (assuming the system
clock underlying Object.wait(long) is accurate).

Fixed-rate execution is appropriate for recurring activities that
are sensitive to absolute time, such as ringing a chime every
hour on the hour, or running scheduled maintenance every day at a
particular time. It is also appropriate for for recurring activities
where the total time to perform a fixed number of executions is
important, such as a countdown timer that ticks once every second for
ten seconds. Finally, fixed-rate execution is appropriate for
scheduling multiple repeating timer tasks that must remain synchronized
with respect to one another.

IllegalStateException - if task was already scheduled or
cancelled, timer was cancelled, or timer thread terminated.

cancel

public void cancel()

Terminates this timer, discarding any currently scheduled tasks.
Does not interfere with a currently executing task (if it exists).
Once a timer has been terminated, its execution thread terminates
gracefully, and no more tasks may be scheduled on it.

Note that calling this method from within the run method of a
timer task that was invoked by this timer absolutely guarantees that
the ongoing task execution is the last task execution that will ever
be performed by this timer.

This method may be called repeatedly; the second and subsequent
calls have no effect.

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